The bonus cloak duration works to a lesser degree with the Dead Ringer due to its innate cloak duration bonus. If the Your Eternal Reward is equipped, its drain penalty is nullified and the cloak duration bonus of the L'Etranger is halved.

Can you cloak with the Dead Ringer?

The Dead Ringer is an unlockable PDA weapon for the Spy. ... Upon taking damage while the Dead Ringer is being held in hand, the Spy will immediately and fully cloak, leaving behind a fake corpse to deceive enemies.

Does L Etranger work with cloak and dagger?

@alexqwx Yes. It applies to all cloaking devices.